1. Field of the Invention
This invention relates to a filtering apparatus and a filtering device for removing suspended solid contained in hot water and a power plant in which the filtering apparatus or the filtering device is installed.
2. Description of the Related Art
Conventionally, in a case of removing suspended solid generated from structural material or piping material by a condenser in a power plant, especially removing iron oxide, hollow fiber filter apparatus made from polyethylene is used before heating feedwater. Moreover, for removing suspended solid in hot water after heating feedwater, a metallic filter or an electromagnetic filter, etc., is used. For example, it is shown in Japanese Patent Disclosure (Koukai) No. H10-339793.
However, by using a filtering apparatus with the filter mentioned above, although a filter or a filtration material which has the mechanism to reduce concentration of the suspended solid is installed for suppressing prevention of heat transmission and corrosion inside of piping, since heat-resistant temperature of the above-mentioned hollow fiber filter apparatus made from polyethylene is about 60 degrees Centigrade, it cannot be applied to a feedwater heater. In addition, the suspended solid which exists in feedwater is also generated in the feedwater heater.
Moreover, a metal filter has subjects that it is chemically unstable, and a used metallic material may begin to dissolve and corrode or form an oxide film, and thus pores of the filter may be blocked. Furthermore, an electromagnetic filter has deferrization performance that is greatly influenced by quality of the suspended solid.
In regard to a general heat-resistant filter module composed of a hollow porosity film, a manufacturing method composed of pouring resins of low viscosity, such as an epoxy resin, a urethane resin, and a silicone resin, into the gap of an outer case and the hollow porosity film and heating the resin to harden is learned. For example, it is shown in Japanese Patent Publication (Koukoku) No. S44-5526, and Japanese Patent Publication (Koukoku) No. S56-40602.
However, even if the hollow porosity film itself is a material having sufficient performance from a viewpoint of heat-resistance and elution, the amount of elution from these resins used as an encapsulant is large and inadequate, and it is difficult to apply these resins to a filter for removing suspended solid from hot water in the temperature of over 100 degrees Centigrade, such as feedwater or heater drain of a power plant whose water quality is highly managed.
In order to solve this problem, it is proposed a method of manufacturing a filter module, with heat-resistance and little elution, made from only fluororesin as the hollow porosity film and to seal the hollow porosity filter with a thermoplastic fluororesin. For example, it is shown in Japanese Patent Publication (Touroku) No. 2993217.
FIG. 21 is a sectional view showing an example of a seal portion of a filter module of the hollow porosity film obtained by the above-mentioned conventional method.
As shown in FIG. 21, in a filter module 104 of the hollow porosity film, a hollow porosity film 102 made from polytetrafluoroethylene (PTFE), which is a kind of fluororesin, is settled inside of an outer case 101 and sealed with polytetrafluoroethylene-hexafluoropropylene copolymer (FEP) to form a support portion 103 of FEP. The outer case 101 can be made from metals, such as stainless steel, PTFE, FEP, etrafluoroethylene-perfluoroalkylvinylether copolymer (PFA), etc. FEP and PFA of fluororesins are suitable for a thermoplastic resin which can be used as an encapsulant.
By this method, since a filter module can be composed of metal and fluororesin only, or fluororesin only, from a viewpoint of necessary condition of heat-resistance and elution required as a filter for dissociating suspended solid contained in the hot water of a power plant, this filter is improved rather than a conventional filter having the above-mentioned problem.
By the way, in order to prevent lowering of heat-conductivity by extraction and precipitation of suspended solid or a dissolution component contained in water to piping, or to prevent vibration of a pump, in the above-mentioned conventional power plant, a filtering apparatus or a desalination equipment is installed to remove suspended solid and ions in the water.
By installing a filtering apparatus composed of the above-mentioned filter made from polyethylene, suspended solid in condensate at low temperature can be removed, however, the filter made from polyethylene is not applicable for removing suspended solid generated in hot water at a temperature of not less than 100 degrees Centigrade because heat-resistant temperature of the polyethylene filter is low.
Moreover, the metallic filter applied to feedwater at a temperature of over 100 degrees Centigrade tends to be chemically unstable and clogged up with deposit of solid substances, and it may pollute water by elution from the filter material in view of standard water quality control, therefore applying the metallic filter to hot water is difficult.
On the other hand, the electromagnetic filter has low filtering performance because it can remove only magnetized suspended solid, and it is difficult for the electromagnetic filter to reproduce the filter for removing dissociated suspended solid.
Furthermore, the filter shown in FIG. 21 cannot avoid heat cycles for treating hot water of a power plant, and the filter module 104 tends to be damaged due to expansion and contraction and exfoliation of the outer case 101 and the support portion 103, because of differences of their materials and their thermal expansion coefficients.
Moreover, in a filter applied as an object for condensate filtering, back wash operation to reproduce hollow fibers which constitute the filter is performed. In this case, by using a fact that gas cannot pass through the film at a pressure which is lower than a bubble point, which is the minimum pressure at which the gas can pass through the film, a method of removing suspended solid deposited on a surface of the film by pressing at certain pressure which is lower than the bubble point to push out water in the upper space of the filtering apparatus is adopted.
On the other hand, a hydrophobic filter applied to such as a power plant must be made hydrophilic by using alcohol, etc. before use. Although the above-mentioned conventional method of back wash can be applied if the bubble point is more than the pressure at the back wash process, if packed material or a film constituting the filter has low bubble point, passing through gas in back wash makes it hydrophobic and thus it needs to be made hydrophilic. Moreover, purification process of the system takes additional time and medical fluid processing is also necessary.
Moreover, the back wash process of the filter used at high temperature cannot be carried out at the high temperature, so it needs cooling process before the back wash process and heating process before restarting. Therefore, it needs to be furnished for cooling and heating, and it additionally takes long time to restart the filter because long cooling time and long heating time is included.
In the above-mentioned conventional filtering apparatus, there are several awaiting solution. That is, the hollow fiber filter made from polyethylene cannot be applied to a feedwater heater because its heat-resistant temperature is about 60 degrees Centigrade, and the metal filter is chemically unstable and its used metallic material dissolves and its pores may be blocked due to corrosion in proportion to forming an oxide film, and the electromagnetic filter has deferrization performance that is greatly influenced by quality of the suspended solid.
Furthermore, the filter shown in FIG. 21 cannot avoid heat cycles for treating hot water of a power plant, and the hollow porosity film 102 tends to be damaged due to expansion and contraction of the outer case 101 and the support portion 103 whose thermal expansion coefficients differ each other, and thus pollution of water quality is occurred due to the base of the filter. And it is difficult to reproduce the filter, thus it is difficult to use as a filter for removing suspended solid contained in hot water of a power plant in which highly management of water quality and long-term stable performance are needed.
And in the conventional back wash method of a filtering apparatus, if the bubble point of packed material or a film constituting the filter is low, making the filter hydrophilic is necessary because passing gases in the back wash process makes the filter hydrophobic. And excessive purification process of the system and medical fluid processing are also needed.
Moreover, it needs cooling before the back wash process and heating before restarting because it cannot back wash the filter used in high temperature circumstance without cooling, so it needs time and additional facilities for cooling and heating and it takes long time up to restarting.
Recently it is found that fluororesin is chemically stable under hot water especially with the temterature of over 150 degrees Centigrade. However, since the fluororesin filter is manufactured by pulling filter base element, if the fluororesin filter contacts with hot water in filtering operation, the fluororesin filter tends to deformed and the permeability of the fluororesin is lowered and damaged because of heat cycle.